Field of the invention: The present invention relates generally to brightness and contrast improvements in compact liquid crystal light valve based video projection systems. More particularly, the invention relates to a compact optical design that retains the color pass band overlap of the copending application referenced above.
The above identified co-pending application, describes a projection system with three color primary light paths, three light valve modulators and a single projection lens. This system has a low f/# and color passband overlap to improve brightness efficiency. However, the projector described in Haven, et. al. requires long optical paths due to the coplanar geometry of the optics together with the use of CRTs as modulators for the liquid crystal light valves. The long necks of the CRT modulator intrude into the illumination or projection light path unless long optical paths are used. But to do this causes the optical elements to be large and expensive, and it adds overall size and weight to the projector. The coplaner geometry also requires inefficient waveplates to rotate or retard the light to allow wavelength passband overlap.
U.S. Pat. No. 4,749,259, issued to Ledebuhr, describes a single-lens light valve projection system on one plane. The optical system is immersed in a bath of high refractive-index fluid to form index-matched, color selective polarizing beam splitters. The fluid also has the effect of making the geometric path length shorter.
U.S. Pat. No. 4,425,028, issued to Gagnon, describes a compact projector system. This optical design uses plate prepolarizing beam splitters immersed in high refractive index fluid. The advantage of this design is that it allows high contrast and is compact in size and configuration.
U.S. Pat. No. 4,826,311, issued to Ledebuhr, teaches a single light-valve, multiple projection lens projection system on two levels, but requires a complex and expensive arrangement of prisms and periscopes.
U.S. Pat. No. 4,191,456, issued to Hong, teaches compact full-color, single projection system where the projection optics and light valve modulators are on a first elevation and the illumination optics are on a second elevation.
All described prior art utilizing a single projection lens rejects one polarization state by means of a reflective polarizing prism before the primary colors are segregated by dichroic filters. Therefore, there can be no overlap in the passbands and brightness efficiency will be lower than that of the present invention. In addition, none teach configurations useful in eliminating wave retarding plates.